<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>CSS揭秘</title>
  <style>
    .border-shadow{
      width:120px;
      height:100px;
      background:lightgreen;
      box-shadow:0 0 0 10px #655;
      margin:30px auto;
    }
  </style>
</head>
<body>
  
  <div class="border-shadow"></div>
</body>
</html>